The next generation of electrodes for neural prostheses will require arrays of microelectrodes for highly selective stimulation and monitoring of the stimulated environment. The goal of this program is to develop microelectrode structures that have good long term stability in biological environments, are simple to produce, and are compatible with a new generation of electrochemically reversible, high charge capacity electrode materials such as iridium oxide. It is the goal of the proposed work to develop microband electrodes for both active and passive prosthetic functions. These electrodes are formed from a thin film of vacuum deposited metal sandwiched between two biocompatible insulators (e.g., Si3N4 or parylene). Electrode areas in the range 10-5 cm2 to less than 10-10 cm2 are readily achieved. The goal of Phase I is to demonstrate a band electrode based on Ir metal that is "activated" electrochemically to form iridium oxide at the electrode/electrolyte interface. Experiments in Phase I will determine the fabrication conditions and substrate materials leading_to the highest stability of the band structure. These electrodes will constitute a new-lower limit in area for activated iridium, and the pulsed capacity under typical intracortical conditions will be characterized and compared to results for macroscopic structures. Demonstration of feasibility of the band structure in Phase I will lead to Phase II investigations of multielectrode arrays of band electrodes of various materials designed for stimulation, recording and local chemical sensing of (e.g.) pH and glucose.